Typical gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, one or more pumps, and a main fuel supply line. The pumps draw fuel from the tank and deliver pressurized fuel to the fuel manifolds in the engine combustor via the main supply line. The main supply line may include one or more valves in flow series between the pumps and the fuel manifolds. These valves generally include at least a fuel metering valve and a pressurizing-and-shutoff valve downstream of the main metering valve. In addition to the main supply line, many fuel supply systems also include a bypass flow line connected upstream of the fuel metering valve that bypasses a portion of the fuel flowing in the main supply line back to the inlet of the one or more pumps, via a bypass valve. The position of the bypass valve, if included, may be controlled to maintain a substantially fixed differential pressure across the fuel metering valve.
A redundant channel engine controller such as, for example, a Full Authority Digital Engine Controller (FADEC), controls the operation of the engine and the fuel supply system. In particular, each of the redundant channels in the engine controller receives input parameters from the engine and aircraft and a thrust setting from the pilot. In response to these inputs, the engine controller modulates the position of at least the fuel metering valve to control the fuel flow rate to the engine fuel manifolds to maintain the desired thrust. In many instances, the position of the fuel metering valve, and thus fuel flow, is controlled based on a nominal fuel flow calibration curve that may be built into, or programmed into, the engine controller.
Although the above-described system and method for controlling fuel flow is generally safe, reliable, and robust, it does suffer certain drawbacks. For example, the fuel metering accuracy is presently limited to the mechanical tolerances of the fuel metering valves. Although accuracy could be enhanced by storing individual fuel metering valve fuel control calibration data directly on the fuel metering valves it is presently not possible to do so using a conventional electronic storage device, such as an EEPROM. This is because the fuel metering valve typically reaches temperatures during engine operations that are too high for such conventional electronic storage devices to operate.
Hence, there is a need for a circuit and method of increasing the fuel metering accuracy of gas turbine engine fuel metering valves, as well as the position control accuracy of various other valves, without further reliance on mechanical tolerances and by using calibration data that is stored on the valve. The present invention addresses at least these needs.